This invention relates to a microminiature semiconductor capacitive acceleration sensor which detects, for example, an acceleration state, a joggling state of an automobile and processes a detected signal so as to be used in various controls.
A technique has been developed in which a silicon layer made of, for example, polysilicon, and a sacrifice layer made of, for example, PSG (Phospho Silicate Glass) are formed so as to constitute a multilayer structure, the multilayer structure is processed by a micromachinning technique, and then the sacrifice layer is removed away by hydrofluoric acid or the like. Hereinafter, this technique is referred to as "multilayer micromachinning technique". A microminiature semiconductor capacitive acceleration sensor has been developed with using such a technique.
FIG. 5 is a perspective view showing an example of a conventional semiconductor capacitive acceleration sensor which is produced with using the multilayer micromachinning technique. The sensor comprises: a semiconductor substrate 1; a supporter 21 which is made of polysilicon and disposed on the upper face of the semiconductor substrate 1 through an insulating layer 1A made of silicon oxide or the like; beams 22A and 22B each of which has one end perpendicularly connected with the supporter 21, which horizontally elongate in parallel, and which have the same length; a movable electrode 23 which is connected with the other ends of the beams 22A and 22B, and which horizontally elongate; a stationary electrode 31 which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A, so as to be separated from the lower face of the movable electrode 23 by a predetermined distance; supporters 42A and 42B which are disposed on the upper face of the silicon substrate 1 through the insulating layer 1A; and a stationary electrode 41 which is connected at the periphery with the supporters 42A and 42B, and which is disposed so as to be separated from the upper face of the movable electrode 23 by a predetermined distance. A terminal M is drawn out from the movable electrode 23 through the beams 22A and 22B and the supporter 21, a terminal S.sub.1 from the stationary electrode 31, and a terminal S.sub.2 from the stationary electrode 41 through the supporter 42B.
The polysilicon is doped with an impurity so that the specific resistance is reduced to, for example, about 1 .OMEGA.cm or the polysilicon is conductive. It is a matter of course that single crystal silicon which is doped with an impurity so as to become conductive may be used in place of polysilicon. However, a sensor using polysilicon can be produced at a lower cost than that using single crystal silicon (this is also applicable to the sensors described below).
The semiconductor capacitive acceleration sensor operates in the following manner: When acceleration is applied to the movable electrode 23 in the vertical direction, the movable electrode 23 receives a force in the vertical direction so that the beams 22A and 22B is bent while their ends connected with the supporter 21 function as fulcrums, whereby the movable electrode 23 is rotated in one of the directions of arrows P. For example, the rotation of the movable electrode 23 causes the distance between the movable electrode 23 and the stationary electrode 31 to be reduced, thereby increasing the electrostatic capacity between the electrodes, and, in contrast, the distance between the movable electrode 23 and the stationary electrode 41 to be increased, thereby decreasing the electrostatic capacity between the electrodes. The values of these electrostatic capacities are respectively obtained through the terminals M and S.sub.1, and M and S.sub.2, and then subjected to a signal processing by a differential amplifier or the like, thereby detecting the applied acceleration.
FIG. 6 is a perspective view showing another example of a conventional semiconductor capacitive acceleration sensor which is similarly produced with using the multilayer micromachinning technique. The sensor comprises: a semiconductor substrate 1; a supporter 21 which is made of polysilicon and disposed on the upper face of the semiconductor substrate 1 through an insulating layer 1A; a beam 24A which has one end connected with the supporter 21, and which horizontally elongates; a beam 24B which has one end connected with the supporter 21, which has the same length as the beam 24A, and which is oppositely directed; and a movable electrode 23 which horizontally elongates. The movable electrode 23 has a rectangular window which is sifted from the center of gravity of the electrode in one lateral direction, for example, the rightward direction. The other ends of the beams 22A and 22B are respectively connected with the sides of the window which are in the longitudinal direction of the window. The sensor further comprises: a stationary electrode 31 which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A, so as to be separated from one side portion of the lower face of the movable electrode 23 with respect to the beams 24A and 24B (in FIG. 6, the left portion of the lower face) by a predetermined distance; and a stationary electrode 41 which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A, so as to be separated from the other portion of the lower face of the movable electrode 23 (i.e., the right portion of the lower face) by a predetermined distance. A terminal M is drawn out from the movable electrode 23 through the beam 24A (24B) and the supporter 21, a terminal S.sub.1 from the stationary electrode 31, and a terminal S.sub.2 from the stationary electrode 41.
The semiconductor capacitive acceleration sensor operates in the following manner: When acceleration is applied to the movable electrode 23 in the vertical direction, the right and left portions of the movable electrode 23 receive forces in the vertical direction, respectively. Since the left portion is heavier than the right portion, the beams 24A and 24B are twisted while their ends connected with the supporter 21 function as fulcrums, whereby the movable electrode 23 is rotated in one of the directions of arrows Q. For example, the rotation of the movable electrode 23 causes the distance between the movable electrode 23 and the stationary electrode 31 to be reduced, thereby increasing the electrostatic capacity between the electrodes, and, in contrast, the distance between the movable electrode 23 and the stationary electrode 41 to be increased, thereby decreasing the electrostatic capacity between the electrodes. The values of these electrostatic capacities are respectively obtained through the terminals M and S.sub.1, and M and S.sub.2, and then subjected to a signal processing by a differential amplifier or the like, thereby detecting the applied acceleration.
FIG. 7 is a perspective view showing a further example of a semiconductor capacitive acceleration sensor of the prior art which is similarly produced with using the multilayer micromachinning technique. The sensor comprises: a semiconductor substrate 1; supporters 21A and 21B each of which is made of polysilicon and disposed on the upper face of the semiconductor substrate 1 through an insulating layer 1A; beams 24A and 24B which respectively have ends connected with the supporters 21A and 21B, and which elongate horizontally opposingly; a movable electrode 23 which is connected between the other ends of the beams 24A and 24B, and which horizontally elongates; a stationary electrode 31 which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A, so as to be separated from one side portion of the lower face of the movable electrode 23 with respect to the beams 24A and 24B (in FIG. 7, the rear portion of the lower face) by a predetermined distance; and a stationary electrode 41 which is disposed on the upper face of the silicon substrate 23 through the insulating layer 1A, so as to be separated from the other portion of the lower face of the movable electrode 23 (i.e., the front portion of the lower face) by a predetermined distance. A terminal M is drawn out from the movable electrode 23 through the beam 24A and the supporter 21A, a terminal S.sub.1 from the stationary electrode 31, and a terminal S.sub.2 from the stationary electrode 41. The reference numeral 25 designates a weight connected with the rear portion of the movable electrode 23.
The semiconductor capacitive acceleration sensor operates in the following manner: When acceleration is applied to the movable electrode 23 in the vertical direction, the rear and front portions of the movable electrode 23 receive forces in the vertical direction, respectively. Since the rear portion is heavier than the front portion, the beams 24A and 24B are twisted while their ends connected with the supporters 21A and 21B function as fulcrums, whereby the movable electrode 23 is rotated in one of the directions of arrows R. For example, the rotation of the movable electrode 23 causes the distance between the movable electrode 23 and the stationary electrode 31 to be reduced, thereby increasing the electrostatic capacity between the electrodes, and, in contrast, the distance between the movable electrode 23 and the stationary electrode 41 to be increased, thereby decreasing the electrostatic capacity between the electrodes. The values of these electrostatic capacities are respectively obtained through the terminals M and S.sub.1, and M and S.sub.2, and then subjected to a signal processing by a differential amplifier, etc., thereby detecting the applied acceleration.
FIGS. 8A and 8B show a still further example of a semiconductor capacitive acceleration sensor of the prior art which is similarly produced with using the multilayer micromachinning technique. FIG. 8A is a perspective view, and FIG. 8B is a sectional view along the line C--C of FIG. 8A. The sensor comprises: a semiconductor substrate 1; supporters 21A to 21D each of which is made of polysilicon, and which are disposed on the upper face of the semiconductor substrate 1 through an insulating layer 1A so as to be placed at positions corresponding to corners of a quadrilateral; beams 22A to 22D which respectively have ends connected with the respective supporters 21A to 21D, which coincide with each other when the beams are rotated by 90.degree., and which elongate in the diagonal directions of the quadrilateral; a movable electrode 23 which is connected with the other ends of the beams 22A to 22D; a stationary electrode 31 which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A, so as to be separated from the lower face of the movable electrode 23; supporters 42A and 42B which is disposed on the upper face of the silicon substrate 1 through the insulating layer 1A; and a stationary electrode 41 which is connected at a periphery with the supporters 42A and 42B, so as to be separated from one portion of the upper face of the movable electrode 23 (in FIGS. 8A and 8B, the left portion of the upper face) by a predetermined distance. A terminal M is drawn out from the movable electrode 23 through the beam 22A and the supporter 21B, a terminal S.sub.1 from the stationary electrode 31, and a terminal S.sub.2 from the stationary electrode 41 through the supporter 42A.
The semiconductor capacitive acceleration sensor operates in the following manner: When acceleration is applied to the movable electrode 23 in the vertical direction, the movable electrode 23 receives a force in the vertical direction to be moved vertically. For example, the vertical movement of the movable electrode 23 causes the distance between the movable electrode 23 and the stationary electrode 31 to be reduced, thereby increasing the electrostatic capacity between the electrodes, and, in contrast, the distance between the movable electrode 23 and the stationary electrode 41 to be increased, thereby decreasing the electrostatic capacity between the electrodes. The values of these electrostatic capacities are respectively obtained through the terminals M and S.sub.1, and M and S.sub.2, and then subjected to a signal processing by a differential amplifier, etc., thereby detecting the applied acceleration.
In the semiconductor capacitive acceleration sensors shown in FIGS. 5 to 7, when acceleration is applied, the movable electrode is rotated with respect to the stationary electrodes, and the changes of the electrode gaps caused by the rotation of the movable electrodes are detected as the changes of the electrostatic capacities. The amount of the rotation of the movable electrode is not proportional to that of each change of the electrode gap, thereby producing a problem in that the detection output characteristic is not linear. In the semiconductor capacitive acceleration sensor shown in FIGS. 8A and 8B, when acceleration is applied, the movable electrode is vertically moved with respect to the stationary electrode, and the changes of the electrode gaps caused by the vertical movement of the movable electrode are detected as the changes of the electrostatic capacities. The amount of the vertical movement of the movable electrode is proportional to that of each change of the electrode gap, so that detection output shows a liner characteristic. Since the movable electrode is supported by four short beams, however, there is a problem in that the amount of the vertical movement of the movable electrode is so small that the detection sensitivity is low (if the beams are lengthened as they are so as to solve this problem, the device size is increased). In the semiconductor acceleration sensor shown in FIGS. 8A and 8B, moreover, the one stationary electrode (designated by 31 in FIGS. 8A and 8B) and the other stationary electrode (designated by 41 in FIGS. 8A and 8B) which respectively produce electrostatic capacities changing in the manners contradictory to each other have different areas, and hence the values (absolute values) of the electrostatic capacities are different from each other. This causes the circuit for processing signals, for example, the differential amplifier to be complicated, thereby increasing the production cost.